1. Field of the Invention
This invention relates to ionomer compositions having improved high temperature utility and improved moldability compared with standard ionomers of comparable comonomer composition. The ionomer compositions are prepared from acid copolymers polymerized at lower than normal temperatures but at normal pressures. The ionomer compositions may be either blends of ionomer prepared at moderate to very low temperatures with standard ionomer, or ionomer prepared at moderately low temperatures.
2. Description of Related Art
Copolymers of ethylene and an unsaturated carboxylic acid such as (meth)acrylic acid, optionally with another comonomer, and their derived ionomers are well known. These copolymers typically contain at least 50 weight percent and up to about 95 weight percent ethylene. Not unexpectedly, they have some characteristics which reflect crystallinity somewhat similar to that of polyethylene. The polar acid groups in such acid copolymers lead to differences from, and certain advantages, as well as some disadvantages compared with polyethylene itself. When the acid copolymers are neutralized, the resulting ionomers contain ionic bonds which lead to an additional difference, and some advantages over the acid copolymer itself. Ionomers contain effective crosslinking at use temperatures, yet thermoplastic processibility at melt temperatures. Ionomer properties thus display characteristics which reflect a crosslinked nature, and an ionic nature. Ionomers have higher tensile strength, greater clarity, better abrasion resistance and higher stiffness than acid copolymers with comparable melt index (MI) and comonomer level.
The higher the acid level, the higher the degree of ionic character possible, since there are more acid groups to be neutralized with metal cations. Higher levels of neutralization will increase ionic character for a given acid level. Neutralization increases molecular weight (particularly weight-average rather than the underlying number-average chain length) and viscosity. MI decreases on neutralization. Thus the acid copolymers used to make ionomers are polymerized to a much lower molecular weight (higher MI) than typical for acid copolymers (other than those for adhesive use where high MI is the norm), and then neutralized to higher molecular weight (lower MI) via ionic crosslinking. The molecular weight required to achieve good mechanical properties in ionomers is thus achieved, in part, by `ionic` crosslinking rather than by increasing degree of polymerization of linear chains per se. For acid copolymers of given comonomer composition, (i.e., un-neutralized), property improvements come from increasing degree of polymerization
The interspersed copolymerized acid units, however, modify and may reduce the level of crystallinity compared with polyethylene and, unfortunately, reduce the melting point and upper use temperature to well below that of polyethylene itself. Neutralization generally further reduces the freezing point somewhat and may reduce the amount of crystallinity. Increasing the use temperature of ionomeric copolymers, while maintaining their essential ionomer character, has become a holy grail.
Typical commercial ionomers, such as those sold under the trade name Surlyn.RTM. by E. I. du Pont de Nemours and Company, derive from acid copolymers with about 9 to 20 weight percent (meth)acrylic acid comonomer. As normally prepared, both the acid copolymers and their derived ionomers have differential scanning calorimetry (DSC) melting points which are in the region of about 81.degree. to about 96.degree. C., and freezing points in the region of about 40.degree. to about 60.degree. C., depending on the comonomers and amounts of these present. These ranges are considerably below that of low density (branched) polyethylene which is prepared under generally comparable conditions. Such polyethylene for instance, typically has a melting point of about 115.degree. C. as well as a higher freezing point than ionomers. For many uses it would be desirable to increase the melting and freezing point of any particular ionomer in order to maintain mechanical properties to a higher temperature, and to increase the rate of crystallization on cooling respectively. Increasing freezing point and hence crystallization rate can improve certain aspects of melt processability.
U.S. Pat. No. 4,248,990 (Pieski), discloses that the polymerization pressure and temperature both have a strong effect on the stiffness of acid copolymers. Pieski considered polymerization at low pressure using `normal` temperatures, and at low temperature using `normal` (high) pressures alternative options to producing the stiffer polymers of his invention. When low polymerization temperature alone, i.e., at `normal` (high) pressures, was used the Vicat Softening temperature, stiffness, and tensile yield strength increased dramatically for acid copolymers with about 9 to 15 weight percent methacrylic acid, when polymerization temperature was decreased from 250.degree. to 160.degree. C. The increased softening temperature corresponds to an increase in the melting points. This increased temperature was attributed to a change in the randomness of the acid and ethylene groups along the polymer chain. At the same acid level, an increase in the number of acid diads and triads occurs. This results in less break up of the polyethyene sequences in the polymer for a given acid level, and a higher melting point, nearer that of polyethylene.
Pieski discloses, and his data show, that as an alternative to low temperature/normal pressure polymerization, low pressure/normal temperature polymerization also produces more diads and triads. He considered the two different polymerization conditions to be alternative modes of producing polymer of his invention. Temperatures considered suitable to produce the required level of diads for his improved polymers (44 percent of acid as diads) at normal pressures of about 24,000 psi, were 150.degree. to 175.degree. C., but not above, for methacrylic acid and below 140.degree. C. for acrylic acid.
However, low temperature and low pressure may not at all be equivalent alternatives. Based on analogy with polyethylene polymerization, at lower polymerization temperatures, less short chain branching occurs, and this also contributes to higher crystallinity and higher melting point. By contrast, polymerization at low pressure at normal temperatures produces higher levels of short chain branching and hence lower crystallinity--just the opposite of what is required for high temperature behavior. Interestingly, Pieski's data show only slightly higher stiffness for low pressure polymerization, and softening temperature data are entirely absent. Nevertheless, Pieski appears to consider the two modes equivalent. In contrast to Pieski's theories of the all importance of sequence distribution, as a result of the present invention, it is now believed that low branching is at least equally, and probably more important. As a result, the low pressure polymerization mode of Pieski is specifically excluded in the present invention.
There is a very significant decrease in polymer productivity when employing low temperature polymerization. Heat evolved from the polymerization, which will be proportional to the polymerization rate, will determine polymerization temperature for a given monomer feed temperature, when polymerization is run, as it typically is, under largely adiabatic conditions. The temperature difference between feed and polymerization temperature will thus be a measure of polymerization rate. Thus, very generally, for a 40.degree. C. feed, productivity can be reduced to only 135/210 of that for normal polymerization, which is a reduction of about 34 percent, when the polymerization temperature is reduced from 250.degree. to 175.degree. C. Pieski's maximum temperature of about 175.degree. C. represents a restriction which corresponds to a rather uneconomical process relative to that for normal polymerization.
A further problem with low temperature polymerization of acid copolymers is that phase separation of monomer and polymer can occur. Normal polymerization conditions of high pressure and high temperature allow polymerization in one phase. Phase separation is also more acute at higher acid levels, even at normal polymerization temperatures, but particularly at low polymerization temperatures. When phase separation occurs, non-uniform polymerization results.
The concept of blending a low melting point resin with a high melting point resin is well known. Blends of standard ionomers, with their low melting point, with polyethylene with its much higher melting point, are however somewhat incompatible and as a result have certain poorer properties including lower melt strength and loss of clarity. While commercial compositions which are blends of ionomer and a major portion of polyethylene (high density) do exist, their properties are substantially different from those being sought here, which are essentially those of a pure ionomer.
Blending different ionomers or ionomers with acid copolymers is also well known, and for typical copolymers which have acid levels of 9 weight percent and above, incompatibility is not a problem. In addition, ions are believed to be significantly labile so that, even when ionomers have different ions, different acid levels, different acid monomers, and even third monomers, as well as different levels of neutralization, all ions present will become essentially randomly distributed and associated will all acid groups present in the blend. The ions will be fairly randomly distributed throughout the mix of (chemically) differing underlying polymer chains.
Ionomer blending has taken on particular importance in certain end uses such as golf ball materials. Thus, U.S. Pat. No. 5,397,840 (Sullivan et al.) discloses blends of ionomers and acid copolymers for golf ball cover materials. Many similar patents disclose ionomer blends. However, in all these cases, there is no disclosure of blends where the acid copolymers, from which the ionomer components are derived, are prepared under vastly different polymerization conditions.
There is a need for ionomer compositions which maintain properties to even modestly higher temperature levels, and have improved processability characteristics, yet which can be prepared without undue sacrifice in productivity.